A magnetic field measurement apparatus utilizing light beams includes, for example, apparatus configured to measure fine magnetic fields such as magnetic fields from hearts (magnetocardiogram) or magnetic fields from brains (magnetoencephalogram), and is expected to be applied to medical image diagnostic devices. A medium which causes polarization in magnetic moment is used for measurement of the magnetic fields. The medium used here includes a solid element such as diamond provided with lattice defect by nitrogen, and a gas cell in which gas such as alkali metal atoms is encapsulated. Energy of the atoms in the element is excited depending on the magnetic field by irradiation of the element with a pump beam, and a plane of polarization of a probe beam passing through the element is rotated by a magnetooptic effect. The magnetic field measurement apparatus measures the angle of rotation of the plane of polarization as magnetic field information. In Patent Document 1, a magnetic field gradient magnetometer including a transverse light beam pumping system combined thereto is described.